The use of DC-DC power converters is wide spread in many important industries including those associated with larger telecommunication and computer installations. The converters are often expected to operate reliably over a variety of load and temperature conditions. The converters usually transform an input DC voltage by converting it first to an AC signal, passing it through a transformer and then rectifying it to provide the desired value of output DC voltage. By employing various switching techniques, the power density of the converters may be increased.
Most DC-DC converters are also expected to operate properly over widely varying values of load current. Many DC-DC converters are designed to supply a rated value of load current while maintaining a highly regulated output voltage. As the load current increases beyond the rated value, the output voltage is driven to zero to protect the converter from heat induced failure. Protection circuits are designed to protect the power rectifiers and other components of the converter from overly large load currents that may cause permanent component failure. Wide temperature operating requirements exacerbate the problems of over-voltage and over-current protection techniques, since the active circuit components of the converter have electrical characteristics that vary with temperature.
For example, the ON resistance of a power MOSFET employable as the power switch in a power converter varies directly with temperature. In the operating range of -40 degrees Celsius to 125 degrees Celsius, which is an operating range usually required for outdoor environments, the ON resistance of a typical MOSFET device may change more than 100%. In conjunction therewith, the ON voltage of the device, for a given current, also varies with temperature. If the ON voltage of the device is being used to provide current sensing in the converter, then a significant shift in the current protection point change may result due, in pertinent part, to the temperature-dependent characteristics of the device. Also, the protection system is further complicated by the fact that the sensed voltage of the switch is usually very noisy. The above described attributes, therefore, usually dictate that while the sensed voltage characteristic may be employed in protection systems, it is not the characteristic of choice for current-mode control of the converter. Additionally, while other sensing techniques involving bulky transformers and resistor networks may be employed, it is at the expense of higher costs and lossy control systems.
Accordingly, what is needed in the art is a system and related method adapted to sense and compensate for temperature effects on components of a power converter that effectively enhances current protection accuracy and stabilizes converter control.